ABSTRACT
This chapter explains the interacting role of defects and composition in non-stoichiometric compounds Materials which have a composition range are called non-stoichiometric compounds. The appearance of non-stoichiometry will be closely associated with the presence of substitutional or interstitial defects in the structure. Non-stoichiometric compounds are important because the composition variation endows the compounds with useful and often fascinating properties. Moreover, these can be modified by varying the relative proportions of the atomic constituents and in controlling the types of defects present. An X-ray powder photograph yields a measurement of the average unit cell dimensions of a material, and, for a non-stoichiometric compound, this invariably changes in a regular way across the phase range. In a similar way, the density of a material gives the average amount of matter in a large volume of material, and for a non-stoichiometric phase this also varies across the phase range.
